Percussion drill bits are widely used both for drilling relatively shallow bores in hard rock and for creating deep boreholes. For the latter application, drill strings are typically used in which a plurality of rods are interconnected to advance the drill bit and increase the depth of the hole. In ‘top hammer drilling’ a terrestrial machine is operative to transfer a combined impact and rotary drive motion to an upper end of the drill string whilst a drill bit positioned at the lower end is operative to crush the rock and form the boreholes. In down-the-hole hammer (DTH) drilling the impact is delivered not through the upper end of the string, but by a hammer directly connected to the drill bit within the hole.
Fluid is typically transported through the drill string and exits at the base of the borehole via passageways in the drill head to flush the rock cuttings from the boring region to be conveyed backward around the outside of the drill string. Examples of percussive drill bits are disclosed in U.S. Pat. Nos. 3,346,060; 4,051,912; US 2010/0108398; WO 93/06332; U.S. Pat. Nos. 4,716,976 and 6,789,632.
Conventionally, the exit end of the flushing passageways are circumferentially spaced apart at the front face and comprise circular cross sectional profiles. In some instances the passageways emerge within grooves recessed at the front face to facilitate the radially outward and axially rearward transport of the cut rock fragments. However, existing drill bits are disadvantageous in that certain regions of the front face do not receive the flushing fluid and are therefore susceptible to debris accumulation. This reduces the forward drilling rate and increases the frictional wear of the bit and in particular the cutting inserts. Accordingly, what is required is a drill bit that addresses these problems.